THIS INVENTION relates to a manganese dioxide-based material. It relates also to an electrochemical cell incorporating the material.
According to a first aspect of the invention, there is provided a manganese dioxide-based material which is highly crystalline, chemically prepared, has a predominantly ramsdellite structure, and has a powder X-ray diffraction pattern (CuK.sub..alpha. radiation) in which the ratio of a [110] peak height to a [201] peak height is at least 0.6:1.0.
The material will be used primarily in electrochemical applications, typically as an electrode material in an electrochemical cell having an electrochemically conducting anode, an electrochemically conducting cathode, and an electrochemically insulating electrolyte separating the anode from the cathode.
The material can hence be used as a positive electrode or cathode material in a cell having aqueous or non-aqueous electrolytes, for example those employing a zinc or lithium anode or negative electrode, respectively. It is believed that it will find particular, but not necessarily exclusive, application as positive electrode material in primary or rechargeable lithium cells.
The material may also comprise a minor proportion of .beta.-MnO.sub.2, ie MnO.sub.2 having a rutile structure, as an intergrowth in combination with the predominantly ramsdellite structure.
The material may further comprise a minor proportion of lithium or hydrogen to stabilize the ramsdellite structure. In this regard, the ramsdellite-MnO.sub.2 need not necessarily be a stoichiometric compound in which the Mn:O ratio is 1:2 so that the oxidation state of the manganese ions is 4.0, but may be one in which the Mn:O ratio deviates slightly from 1:2 such that the oxidation state of the manganese ions is less than 4.0 but greater than 3.8 preferably greater than 3.8.
In the powder X-ray diffraction pattern, the ratio of the [110] peak height to the [201] peak height may be at least 0.8:1.0, typically about 1.0:1.0, which indicates a high degree of crystallinity and single-phase character of the ramsdellite manganese dioxide structure. The [110] peak may occur at about 22.degree.2.theta., while the [201] peak may occur at about 37.degree.2.theta.. In the powder X-ray diffraction pattern, the ratio of the [110] peak height to a [221] peak height, which may occur at about 56.degree.2.theta. may be at least 1.2:1.0, also indicating the high degree of crystallinity of the ramsdellite manganese dioxide structure. The ratio of the [110] peak height to the [221] peak height may be about 1.4:1.0. The [110] peak may have a peak width, at half the peak height, of less than 2.degree.2.theta., eg about 1.5.degree.2.theta., which further indicates the high degree of crystallinity of the ramsdellite manganese dioxide structure.
The material may be prepared by reacting a lithium-manganese-oxide compound with concentrated acid. The acid may be sulphuric acid, and its concentration may be at least 2M. It has been found that the use of concentrated acid to digest the lithium-manganese-oxide compound, results in a highly crystalline ramsdellite-MnO.sub.2 structure being synthesized. It is believed that an advantage of the high degree of crystallinity in the ramsdellite structure or phase is that the structural integrity of the ramsdellite-MnO.sub.2 structure on cycling in rechargeable Li/ramsdellite-MnO.sub.2 electrochemical cells may be superior to that obtained from known .gamma.-MnO.sub.2 electrodes, such as electrolytically prepared MnO.sub.2 (`EMD`). A further advantage of the ramsdellite-MnO.sub.2 structure is that it may offer a higher initial discharge capacity for primary cell applications compared to known chemically prepared MnO.sub.2 (`CMD`) materials and EMD products.
The lithium-manganese-oxide precursor compound may be selected from stoichiometric spinel compounds such as LiMn.sub.2 O.sub.4, or defect spinel compounds such as those found in the system Li.sub.2 O.yMnO.sub.2, for example, Li.sub.2 Mn.sub.4 O.sub.9 (y=4) or Li.sub.2 Mn.sub.3 O.sub.7 (y=3).
These precursor compounds can typically be synthesized by reaction of manganese carbonate, MnCO.sub.3, and lithium carbonate, Li.sub.2 CO.sub.3, in the required ratios and at predetermined temperatures, for example
______________________________________ Li.sub.2 CO.sub.3 + 4MnCO.sub.3 800.degree. C. 2LiMn.sub.2 O.sub.4 + 5CO.sub.2 .fwdarw. air Li.sub.2 CO.sub.3 + 4MnCO.sub.3 .about.400.degree. C. Li.sub.2 Mn.sub.4 O.sub.9 + 5CO.sub.2 .fwdarw. air Li.sub.2 CO.sub.3 + 3MnCO.sub.3 .about.400.degree. C. Li.sub.2 Mn.sub.3 O.sub.7 + 4CO.sub.2 .fwdarw. air ______________________________________
The lithium-manganese-oxide spinel precursor compounds may be digested in concentrated sulphuric acid, for example, 2.6M H.sub.2 SO.sub.4, at elevated temperature, for example, at about 95.degree. C., for several hours, to leach out effectively all the lithium and to generate the highly crystalline ramsdellite-MnO.sub.2 phase. For example, when taken to completion the ideal reactions can be represented by EQU 2LiMn.sub.2 O.sub.4 .fwdarw.3MnO.sub.2 +MnO+Li.sub.2 O EQU Li.sub.2 Mn.sub.4 O.sub.9 .fwdarw.4MnO.sub.2 +Li.sub.2 O EQU Li.sub.2 Mn.sub.3 O.sub.7 .fwdarw.3MnO.sub.2 +Li.sub.2 O
It should be noted, however, that in practice the final ramsdellite phase may contain a small amount of lithium or hydrogen which, it is believed, may serve to stabilize the structure.
The ramsdellite-MnO.sub.2 phase when made by the method according to the invention will usually contain a small amount of water that is normally associated with the surface of the MnO.sub.2 particles or with grain boundaries. This water content is important when the ramsdellite-MnO.sub.2 is to be used as an electrode in aqueous cells, for example, those using zinc anodes. However, when it is to be used in lithium cells, the ramsdellite-MnO.sub.2 phase must be heated to 100.degree. C. or higher to remove water therefrom. In this respect the ramsdellite-MnO.sub.2 phase of the present invention has been found to be remarkably stable to 250.degree.-300.degree. C.; however heat-treatment above 300.degree. C. causes a transformation to a .beta.-MnO.sub.2 (rutile-type) structure.
Instead, the ramsdellite-MnO.sub.2 phase may be dehydrated at elevated temperature, eg 200.degree.-400.degree. C., preferably 300.degree.-370.degree. C., in the presence of a lithium salt such as LiOH, LiNO.sub.3 or Li.sub.2 CO.sub.3, to generate lithium-stabilized ramsdellite phases, optionally in the presence of additional lithium manganese-oxide phases such as spinel phases that may be produced as a by-product of the reaction. Although the exact compositional range of these phases has not been determined, it is believed that the overall composition can be represented by Li.sub.2x MnO.sub.2+x with 0.ltoreq.x.ltoreq.0.2. It should also be noted that the MnO.sub.2 component in the Li.sub.2x MnO.sub.2+x ramsdellite-related phases need not be stoichiometric, but can be slightly oxygen deficient such that the oxidation state of the manganese cations is slightly less than 4.0.
While the material has been described as being suitable for use as an electrode material, it is believed that it can also be used in catalytic applications.
According to a second aspect of the invention there is provided a manganese dioxide-based material which is highly crystalline, has a predominantly ramsdellite structure, and has a powder X-ray diffraction pattern (CuK.sub..alpha. radiation) in which the ratio of a [110] peak height to a [201] peak height is at least 0.6:1.0 and in which the [110] peak has a peak width, at half the peak height, of less than 2.degree.2.theta..
The material according to the second aspect of the invention may also be chemically prepared as hereinbefore described, and may have relative peak heights and peak widths as hereinbefore described.
According to a third aspect of the invention, there is provided an electrochemical cell, which comprises
an electronically conducting anode; PA1 an electronically conducting cathode comprising a highly crystalline, chemically prepared, manganese dioxide having a predominantly ramsdellite structure and having a powder X-ray diffraction pattern (CuK.sub..alpha. radiation) in which the ratio of a [110] peak height to a [201] peak height is at least 0.6:1.0; and PA1 an electronically insulating electrolyte separating the anode from the cathode. PA1 an electronically conducting anode; PA1 an electronically conducting cathode comprising a highly crystalline manganese dioxide having a predominantly ramsdellite structure and having a powder X-ray diffraction pattern (CuK.sub..alpha. radiation) in which the ratio of a [110] peak height to a [201] peak-is at least 0.6:1.0 and in which the [110] peak has a peak width, at half the peak height, of less than 2.degree.2.theta.; and PA1 an electronically insulating electrolyte separating the anode from the cathode.
The cell may be a primary or a secondary, ie rechargeable, cell, and the electrolyte may be aqueous or non-aqueous with the anode then, for example, being zinc or hydrogen, in the case of an aqueous electrolyte, or lithium in the case of a non-aqueous electrolyte.
According to a fourth aspect of the invention, there is provided an electrochemical cell, which comprises
The manganese dioxide may be as hereinbefore described, and in particular may have a powder X-ray diffraction pattern as hereinbefore described.